Scale changes when applying curve modifier
If you've scaled your curve and applied its scale, the radius of its vertices is no longer 1. So select your curve, go in Edit mode, select all the vertices, and W > Set Curve Radius (value 1 by default, that's what you want, but you can change it on the bottom of the T panel), or in the Tools panel of your 3D view (displayed with T on the left of the 3D view) go in Tools > Curve Tools > Set Curve Radius and choose a value of 1.